US3484303A - Steel surface treatment - Google Patents

Description

-Dea; I6, 1969 'R. BABC'OCK 3,484,303

STEEL SURFACE TREATMENT Filed Dec. 20, 1967 INVENTOR ROGER S. BABCOCK av I 4 ATTORNEY United States Patent 3,484,303 STEEL SURFACE TREATMENT Roger S. Babcock, Chatham, N.J., assignor to Union Carbide Corporation, a corporation of New York Continuation-impart of application Ser. No. 481,329, Aug. 20, 1965. This application Dec. 20, 1967, Ser. No. 692,050

Int. Cl. C23g /00 US. Cl. 148-4 12 Claims ABSTRACT OF THE DISCLOSURE A process for desurfacing steel, particularly semi-finished steep shapes. The process consists of immersing the steel shape in a bath of molten iron containing at least nominal amounts of carbon for a sufiicient time to cause thermochemical removal of scale and/or metal from the outer surface of the steel shape.

This application is a continuation-in-part of application Ser. No. 481,329, filed Aug. 20, 1965, and now abandoned.

The present invention relates, in general, to the surface treatment of metals, and more specifically to the thermochemical descaling and desurfacing of semi-finished shapes.

The term semi-finished steel shapes is used herein to mean both massive steel products such as ingots, blooms, bars, plates, slabs and the like, as well as shapes of small cross sectional area such as rods and Wires. Semi-finished steel often contains undesirable surface defects and imperfections, such as seams, cracks, scabs, snakes, slag inclusions and scale, that need to be removed prior to subsequent treatment, such as rolling to finished gauge dimensions.

The main object of this invention is to provide a novel process for such removal that is simple, economical, fast, efficient, and highly effective for such purpose.

The process of the invention, as applied to massive semi-finished steel shapes, comprises immersing the shape, which is at least red hot, in a bath of molten iron, such as pig iron or cast iron, for a time suflicient to cause thermochemical removal of a desired thickness of metal from the outer surface of said steel shape, and then removing said shape from said bath. The surface so obtained is clean and free of undesirable oxide and defects, thus well suited for further processing.

The surface will, of course, reoxidize to some degree if exposed to the air while hot. This can be prevented during transfer of the shape to the next processing step, or while cooling to an unreactive temperature, by a variety of methods such as protecting it with an inert gas such as argon, coating it with a protective flux, or keeping it in a vacuum.

For better understanding of the invention reference is made to the drawing in which:

FIGURE 1 is a view mainly in cross section of a steel shape being immersed in a high carbon iron bath; and

FIGURES 2 and 3 are plan views of such shape before and after such immersion.

As shown in such drawings, a semi-finished steel shape having surface defects 12, is immersed in a bath 14 of molten high carbon iron in a furnace 16, by lowering such shape into the bath with the aid of tongs 18. The shape 10 is preferably red hot, such as that P oduced by heating the shape in a soaking pit, and is immersed for a selected period of time such that upon removal the shape is free of surface defects or imperfections. The steel shape is descaled or desurfaced or both by such im 3,484,303 Patented Dec. 16, 1969 mersion. The cleaned shape 10 is then rolled, or otherwise processed.

While FIGURE 1 shows the immersion achieved by dipping, other methods may be used. For example, in treating an elongated shape such as a wire, rod, or a continuously cast billet or slab, it may be advantageous to pass the piece continuously through the bath. This can be accomplished by causing the moving shape to dip down into the bath. Another method is to flow the molten metal across the surface to be treated.

The molten ferrous metal bath need contain only nominal amounts of carbon, that is about 0.04%; such compositions being referred to as low carbon iron baths. It is preferable, however, that the ferrous metal bath contain larger amounts of carbon, specifically, up to about 4.4%. Molten cast iron and pig iron are especially preferred for use as the bath material in the present invention.

The process of this invention is applicable to carbon steels, alloy steels, as well as stainless steels. These terms are intended to have their common customary meaning as defined, for example, in the A.I.S .I. Steel Products Manual. Thus, carbon steel is defined as containing no minimum content for aluminum, boron, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium or zirconium or any other element added to obtain a desired alloying effect, containing a minimum copper content not exceeding 0.40% and a maximum content of: 1.65% maganese, 0.60% silicon and 0.60% copper. The term, alloy steel is used to mean steel in which the maximum for the content of alloying elements exceeds one or more of the following limits: 1.65% manganese, 0.60% silicon, 0.60% copper, or in which the definite minimum quantity of any of the following elements is specified or required within the limits of a recognized field of constructional alloys steels: aluminum, boron, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium or zirconium or any other alloying element added to obtain a desired alloy effect. Stainless steel is defined as steel containing at least 4% chromium. Other alloying elements such as copper, aluminum, silica, nickel and molybdenum may also be present therein.

By means of the present invention, tightly bonded furnace scale may be removed from semi-finished steel shapes which are at a temperature of from about 1500 F. to 2300 F. by immersion in a bath of molten pig iron at a temperature of from about 2200 to about 2800 F. The exposed outer layer of steel may be removed to a predetermined depth by continued immersion for an appropriate length of time. In treating carbon and alloy steel, immersion times of about 30 seconds were found sufficient to remove heavy furnace scale and to clean the steel surface to a condiiton favorable for hot rolling. Stainless steel, however, required about double the immersion time for carbon steel, at comparable bath temperature and composition.

EXAMPLES The following examples illustrate the practice of the invention with different types of steel. In each example a bar of the indicated composition, approximately 1 inch square and 24 inches long, was heated for about 18 hours in an electric furnace at a temperature of 1800 F. in order to heavily scale it. A bath of indicated composition was melted in an induction furnace and brought to the indicated temperature. The hot bar was then immersed to a depth of about 12 inches and held in the bath for a measured period of time, withdrawn, cooled to room temperature and examined for the amount of material removed and for surface quality. In all cases the scale was completely removed, together with the indicated amount of base metal, leaving a clean metal surface suitable for further processing. When cooled in argon the surface remained clean, when cooled in the air the expected amount of reoxidation occurred. The test conditions and results obtained are shown in Table I below.

metals, and reducing action on scale) can be obtained to greater or lesser degrees from other bath ingredients such as silicon and manganese. These are present at significant concentrations (about 0.1 to 1.0% manganese and 0.1 to 3% silicon) in cast iron and pig iron baths and participate, along with carbon, in the scale reduction reactions.

TABLE I Depth 01 Material Bath Composition Removed, Bath Liquidus Inches Percent Percent Percent temp temp., Immersion Test specimen 11 Si time, see. Scale 6 Metal Example:

1 Carbon Steel 4. 4 0.86 0. 95 2, 580 2, 095 30 0. 150 0.037 do 4. 1 0. 35 0. 27 2, 640 2, 100 35 0. 150 0. 040 2. 9 0. 31 1. 04 2, 600 2, 340 30 0. 125 0. 035 2. 9 0. 31 1. O4 2, 605 2, 340 45 0. 125 0. 095 4. 2, 600 2, 130 0. 090 2 0. 03 0. 04 0. 48 O. 04 2, 905 2, 795 0. 1 0. 25 4. 4 0. 86 0. 95 2, 500 2, 095 0. 125 0. 060 4. l 0. 0. 27 2, 660 2, 100 35 0. 125 0. 040 do 2. 9 0.31 l. 04 2,610 2, 340 30 0.125 0. 035 Mayari-R. steel 2. 3 0.86 2, 550 2, 450 30 0.095 4 5 Neg. 304 Stainless 4. 3 .86 0.95 2, 610 2, 065 60 0.090 4 Neg. d 3. 0 0. 27 0. l3 2, 770 2, 340 30 0. 090 0. 16

1 The liquidus temperatures in the above table are computed from the carbon content only, disregarding silicon and manganese.

Taking these into account would give slightly different values.

2 In example 5 the cross section of the test specimen was 3 inches by 3% inches instead of 1 inch square. 3 Mayari-R. steel is a high strength, low alloy, corrosion resisting steel containing about 0.1% C, 0.6% Cu, 0.7% Cr, 0.5% Ni. 4 In examples 10 and 11, although a negligible amount of the underlying base metal was removed, all the scale was removed leaving a clean surface.

5 In example 10 the specimen W2llS preheated at 1,975 F. instead of 1,800 F,

6 Depth of scale prior to remov As shown by Example 6 above, it is possible to condition a steel surface by the process of the invention using a bath having only a nominal carbon content. The liquidus temperature of such a bath is high and the bath temperature Will, in general, be above the melting point of the steel being treated. In this mode of operation the effect tends to be one in which the surface of the immersed piece is melted off. Control can be obtained by proper adjustment of the temperature of the bath and the immersed shape.

Better process control and a better final surface is obtained using a high carbon bath at a temperature below the melting point of the piece being treated. The carbon in the bath lowers its liquidus temperature and increases its solvent power for iron and iron oxide, and thus permits operation of the bath at lower temperature. In this Thus, a preferred procedure utilizes a bath containing significant amounts of carbon and a bath temperature lower than the melting point of the steel being treated. Manganese and silicon may also be present in the bath. By using appropriate amounts of carbon, manganese and silicon, bath liquidus temperatures as low as 2065 F. can be obtained, and bath operating temperatures down to this level are possible. However, it has been found that at bath temperatures below about 2400 F. the rate of thermochemical attack on the steel preheated to 1800 F. is slow. It has also been found that at bath carbon values below about 1% the rate of removal on steel preheated to 1800 F. becomes very slow at bath temperatures below the melting point of the steel being treated. Higher preheat temperatures will permit use of lower bath temperatures and carbon contents.

mode of operation, scale and metal are removed by dis- An important feature of the process is that the rate Solution rather than y meltingp a bath With and extent to which scale and base metal are removed the Carbon-iron eutectic Composition, about 4.3% carby the bath can be readily controlled over sufiiciently bOIl, as a liquidus temperature of about 2065 F. At an wide ranges to accommodate the varying sizes and shapes operating temperature of 2500 F. such a bath will readily of pieces, alloy compositions, scale thickness and metal dissolve car n t l th a m g p int eXCeeding defect depths which are encountered in typical steel mill 2700 F. and iron oxide scale with a melting point exoperations. In general the rate of removal will increase ceeding 2800 F with increasing bath temperature, with increasing concen- In removing scale the carbon promotes not only its trations of carbon, manganese and silicon in the bath, dissolution but also its reduction to metal. The oxides of and with the surface temperature of the immersed shape. important steel alloying agents such as nickel are reduced By suitable adjustment of these parameters it is possible in the same manner as iron oxide. The reduced oxides to preselect an appropriate immersion time for a parare retained in the bath as metal, and, thus, their metal ti l r piece t b treated. values are conserved in readily recoverable form. The Table II below illustrates how the extent of oxide and scale reduction reaction also generates carbon monoxide b tal removal varies with immersion time, at typical in the bath which provides a beneficial stirring effect constant conditions for bath composition, bath temperathrough the action of the rising gas bubbles. ture and test specimen preheat temperature. The test The effects of carbon in the bath (lowered melting specimen in each case was preheated to 1800 F. prior point, increased dissolving power for iron and other to immersion in the bath.

TABLE II Bath Composition Bath Liquidus Immersion Percent Percent Percent temp., temp time, Test specimen 0 F. sec. Remarks 3. 0 0. 20 0. 10 2, 660 2, 340 6 Partial scale removal. 3. 0 0. 20 0. 10 2, 660 2, 340 14 Nearly complete scale removal. 3.0 0. 20 0. 10 2, 660 2, 340 20 Complete scale removal plus A3 metal. 4. 3 0. 86 0. 2, 610 2, 065 30 Partial scale removal. 4. 3 0. 86 0. O5 2, 610 2, 065 60 Complete scale removal. 4. 3 O. 86 0. 05 2, 610 2, 065 Bar completely dissolved.

The data shows that practical immersion times short enough for economy of operation and long to permit to 1800 F. prior to immersion in the bath. Immersion time in each of these examples was seconds.

control are characteristic of the process. It is, of course, desirable to remove enough material to eliminate all the defects, but to avoid excessive removal in the interest of economy.

Table III below shows the effect of bath temperature on rate and extent of oxide and metal removal at substantially constant bath composition, immersion time, and test specimen preheat temperature. The test specimen in each was preheated to 1800 F. prior to immersion in the bath, and immersed for seconds.

In conditioning steel by the dissolution mode as opposed to the melting mode, the trends indicated in Table IV above are important not only for the selection of a suitable initial bath composition but also to an understanding of how the bath action will vary as its composition changes With use. As material is treated in a given-bath, carbon, silicon and manganese are consumed in reducing the scale. Eventually, therefore, it is necessary either to replenish these elements in the bath or replace the depleted bath with a fresh charge of hot metal. As in- TABLE III Bath Composition Bath Liquidus Percent Percent Percent temp., mp., Test specimen 0 Mn F. F. Remarks e i -3? Carbon steel 3.0 0. 27 0. l3 2, 400 2, 340 Partial scale removal.

20 Nickel steel--- 2. 9 0. 31 1. 04 2, 495 350 Complete scale removal. 21 ..do 2. 9 0.31 1. 04 2, 610 2, 350 Complete scale removal plus ,52" metal. 22.- ..do 4. 5 0.86 0. 95 2, 460 2, 065 Partial scale removal. 23 ...do 4. 5 0.85 0. 85 2, 580 2,065 Complete scale removal plus metal. 24 d0 4. 2 8 0- 71 608 2,065 Complete scale removal plus @3 metal,

Since the surface temperature of the solid to be immersed is generally lower than the bath temperature, there will be a chilling effect on the bath in the immediate v1- cinity of the surface. Therefore some bath superheat (excess of actual bath temperature over its liquidus temperature) is needed to ensure proper bath fluidity and solvent action. If the amount of superheat is too low, metal will deposit from the bath onto the solid surface. Conversely, excessive bath temperature may accelerate scale and metal removal beyond the rate of reasonable control. The amount of superheat required will vary with the temperature to which the solid is preheated and also with the size of the steel shape in relation to the volume of the bath, since this will affect the amount of chill. The chilling effect can be minimized by stirring the bath which will tend to raise the surface temperature of the solid more rapidly to a value approaching the bath temperature. Bath chilling can, of course, be eliminated entirely by having the preheat temperature exceed the bath temperature. Obviously, if a bath is to be used for treating a succession of shapes over an extended period of time heat must be supplied to it to maintain the required temperature.

The preheat temperature of the piece to be treated may vary over a wide range depending on other process conditions. In practice, the useful temperatures will encompass the range typically obtained in soaking pits, reheat furnaces and the like used to control the temperature of solid steel. This range is from about 1500 F. to about 2300" F.

Table IV illustrates the effect of varying bath composition on the rate and extent of oxide and metal removed at a typical set of constant conditions for bath temperature and test specimen preheat temperature. The test specimen in each case was carbon steel, and preheated dicated above the most useful range of bath carbon contents lies between the eutectic value (4.3%) and a lower limit of about 1%. Within these limits it is possible to use the bath continuously as it gradually becomes depleted in carbon, silicon and manganese by making suitable upward adjustments in bath temperature or immersion time or both as the depletion occurs. Alternatively the bath can be replenished with carbon, and also with manganese and silicon, if desired, on a more or less continuous basis during use.

An unexpected advantage of the present invention is a reduction in need for subsequent conditioning of sections rolled after treatment. This results from the excellent surface which the treatment provides, and also from the fact that the bath action around cracks and tears tends to flare out the adjacent surface, i.e. to create a broad, smooth depression so that it is in a condition well suited for subsequent rolling and forming. Thus, deep defects are eliminated with a minimum of total metal removal. Another important advantage is that essentially all of the metal removed, and also much of the scale, is retained in the bath as molten metal, and is conveniently and economically recycled to the steelmaking operation by periodically tapping the bath as its volume builds up. The invention therefore avoids the costly loss of metal values incurred by most conditioning processes.

Although the invention as above described is directed primarily toward relatively massive, semi-finished shapes which have had a minimum of prior mechanical working (ingots, blooms, slabs, etc.) it is also applicable to more highly finished shapes such as :rods and wires. A 1 4" diam. medium carbon steel rod with & layer of scale, preheated to 1800 F. was immersed in molten pig iron at 2600 F. for 4 sec. This treatment removed the surface scale plus 0.023." metal.

In treating sections of relatively small cross-section such as wires, rods, bars and the like, it is possible and may be advantageous to eliminate preheating entirely and immerse the pieces cold. This will, of course, require adequate superheat in the bath and an adequate ratio of bath volume to the volume of the immersed piece. For example, a cold carbon steel bar, 1-inch square which was relatively free from scale was immersed to a depth of about 12-inches in a 500 lb. bath of 3% carbon cast iron at a temperature of 2660 F. for 30 seconds. A layer of steel about -inch thick was removed from the surface of the bar. With higher bath temperature it will be possible to condition cold pieces of relatively small cross-section which are covered with scale.

What is claimed is:

1. A process for desurfacing a semi-finished steel shape comprising: immersing said steel shape in a bath of molten iron containing at least nominal amounts of carbon, for a time suflicient to cause thermochemical removal of a desired thickness of metal from the outer surface of said steel shape, and then removing said shape from said bath.

2. The process of claim 1 wherein said steel shape is protected from atmospheric oxidation after removal from said bath until further processing or until cooled to an unreactive temperature.

3. The process of claim 1 wherein said steel shape has a relatively small cross-sectional area.

4. The process of claim 1 wherein said steel shape is at least red hot when immersed into said bath.

5. The process of claim 4 wherein said steel shape is a massive semi-finished steel product.

6. The process of claim 4 wherein the molten bath is at a temperature below the melting point of said semifinished steel shape.

7. The process of claim 6 wherein said molten iron bath contains up to about 4.4% carbon.

8. The process of claim 7 wherein said molten iron bath contains about 0.1 to 1.0% manganese and about 0.1 to 3% silicon.

9. The process of claim 6 wherein the molten iron bath comprises material selected from the group consisting of molten pig iron and molten cast iron.

10. The process of claim 6 wherein the semi-finished steel shape is of carbon steel.

11. The process of claim 6 wherein the semi-finished steel shape is of alloy steel.

12. The process of claim 6 wherein the semi-finished steel shape is of stainless steel.

References Cited UNITED STATES PATENTS 5/1955 Bucknall 148-14 7/1956 Heger et al. 148-121 US. Cl. X.R. 117-114; 134-2, 5

Disclaimer 3,484,303.1i0ge1' S. Babcock, Chathmn, NJ. STEEL SURFACE TREAT- MENT. Patent dated Dec. 16, 1969. Disclaimer filed Aug. 22, 1972, by the assignee, Union Carbide C'orpomtiow. Hereby enters this disclaimer to claims 1-12 of said Letters Patent.

[Ofiio'ial Gazette May 1,1973]

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